Native tropomyosin from normal myocardium will be isolated from myofibrils free from mitochondrial, sarcoplasmic and sarcolemmal contamination. The extraction procedure does not involve organic solvent treatment, and the protein will be extracted at low ionic strength. Native tropomyosin isolated by this procedure will be characterized in terms of its ability to bind Ca ion and Mg ion ATPase of desensitized actomyosin in the presence of EGTA. Endogenous protein kinase associated with the native tropomyosin and its properties will be determined. The enzyme will be compared with free cytoplasmic protein kinase. Native tropomyosin will be fractionated into components by hydroxylapatite column chromatography. Cardiac troponin will be further fractionated into components. Phosphorylation of tropomyosin, troponin and its components will be studied in the absence and presence of protein kinase with and without cyclic AMP or phosphorylase b kinase. The properties of phosphorylated proteins will be compared with those of nonphosphorylated proteins. Regulatory proteins from experimentally induced ischemic hearts will be isolated and phosphorylation experiments will be carried out as described for controls. A possible defect in cyclic AMP-dependent phosphorylation of regulatory proteins isolated from ischemic myocardium will be examined.